To explore
and work in space, human beings must take their environment with them
because there is no atmospheric pressure and no oxygen to sustain life.
Inside the spacecraft, the atmosphere can be controlled so that special
clothing isn't needed, but when outside, humans need the protection of
a spacesuit.

Earth's atmosphere
is 20 percent oxygen and 80 percent nitrogen from sea level to about 75
miles up, where space begins. At 18,000 feet, the atmosphere is half as
dense as it is on the ground, and at altitudes above 40/000 feet, air
is so thin and the amount of oxygen so small that pressure oxygen masks
no longer do the job. Above the 63,000-foot threshold, humans must wear
spacesuits that supply oxygen for breathing and that maintain a pressure
around the body to keep body fluids in the liquid state. At this altitude
the total air pressure is no longer sufficient to keep body fluids from
boiling.

Spacesuits
for the space shuttle era are pressurized at 4.3 pounds per square inch
(psi), but because the gas in the suit is 100 percent oxygen instead of
20 percent, the person in a spacesuit actually has more oxygen to breathe
than is available at an altitude of 10,000 feet or even at sea level without
the spacesuit. Before leaving the space shuttle to perform tasks in space,
an astronaut has to spend several hours breathing pure oxygen before proceeding
into space. This procedure is necessary to remove nitrogen dissolved in
body fluids and thereby to prevent its release as gas bubbles when pressure
is reduced, a condition commonly called "the bends."

Spacesuits
designed for the space station era will be pressurized to 8.3 psi; therefore,
the pre-breathing period will be shortened or diminished.

The spacesuit
also shields the astronaut from deadly hazards. Besides providing protection
from bombardment by micrometeoroids, the spacesuit insulates the wearer
from the temperature extremes of space. Without the Earth's atmosphere
to filter the sunlight, the side of the suit facing the Sun may be heated
to a temperature as high as 250 degrees Fahrenheit; the other side, exposed
to darkness of deep space, may get as cold as -250 degrees Fahrenheit.

Astronauts
of the space shuttle era have more than one wardrobe for space flight
and what they wear depends on the job they are doing.

During ascent
and entry, each crewmember wears special equipment consisting of a partialpressure
suit, a parachute harness assembly, and a parachute pack. The suit, consisting
of helmet, communication assembly, torso, gloves and boots, provides counterpressure
and anti-exposure functions in an emergency situation in which the crew
must parachute from the orbiter. The suit has inflatable bladders that
fill it with oxygen from the orbiter. These bladders inflate automatically
at reduced cabin pressure. They also can be manually inflated during entry
to prevent the crewmember from blacking out. Without the suit pressing
on the abdomen and the legs, the blood would pool in the lower part of
the body and cause a person to black out as the spacecraft returns from
microgravity to Earth's gravity. The partial-pressure suit and equipment
will support a crewmember for a 24-hour period in a life raft in case
of an egress over water.